1. Field of the Invention
Advanced composites of layers of resin impregnated fibers are rapidly emerging as a primary material for use in near term and next generation aircraft structures as they provide greater structural efficiency at lower weights than equivalent metallic structures. These materials are expected to revolutionize the entire airframe industry within the next few years. The impact on the design and manufacture of near term and next generation high performance aircraft has been and will be significant. Based on the trends to date, the structures of the next generation aircraft could contain over 65 percent advanced composites. To meet these commitments, airframe manufacturers will be obligated to invent novel methods to reduce cost of assembling these composite structures.
Composite structures assembled by existing technology are formed by the lay-up of individual sheets to form a multi-ply or laminate structure prior to curing into a unified laminate component. The respective single-ply sheets of elongated fibers impregnated with partially cured resin have the fibers oriented in a given direction. The fibers may be boron, glass, kevlar or graphite. These elongated fibers are oriented in a first direction and impregnated with an epoxy, plastic or polyamide resin, to form matrices. The individual sheets are layed-up, one on the next, with the fibers oriented at different orientations one to the other. Further, the sheets are arranged along one reference line with the edge of the second sheet displaced from the reference line by a short distance, and following sheets displaced from the edge of the preceding sheet by similar amounts. This relative displacement of the edges provides for structural drop-off of loads relative to the several sheets. Thus, assembly of a composite structure requires that each sheet be layed-up individually with the fibers at different orientations. This lay-up procedure is therefore seen to require significant man-hours to complete the lay-up of a multi-ply composite structures.
A structural drop-off of a sheet occurs where the elongated unidirectional fibers of that sheet cease to carry a load, whereby the forces or loads are transferred into the fibers of adjacent sheets through the binder or matrix bonding the sheets into a composite assembly. When structural drop-offs all end along a common line, high stress concentrations occur which sometimes cause separations between the plies. This separation occurs as the sheer forces, imparted between individual sheets of the composite, cause a failure of the binder or matrix; effectively, delamination of the sheets. Therefore, multi-ply composite laminates require plies of unidirectionally oriented fibers pre-impregnated with resin to be oriented and positioned individually so that applied loads "drop-off" gradually from one ply to another, thereby minimizing interlaminar load transfer.
The time consumed in positioning each ply individually by the composite lay-up technician is the major problem in the manufacture of a structural part using unidirectional composite material preimpregnated with resin in drop-off areas.
The present invention overcomes the problem by replacing two separate plies of unidirectional material with one, feather cut, preplied sheet of cross-plied material in which the fibers are oriented on a bias one to the other, thereby imparting dimensional stability to the lay-up, providing a structural drop-off, and reducing lay-up time.